Permeability of Lron and Steel. 15 
we have 
GB =47 = fb. 
Now 47Cn is the total magnetic potential exerted on the ring 
by the magnetizing coil ; whence 
potential 1 
a 
which is what I call the magnetic resistance*. is then cal- 
culated really both by Rowland and myself from the formula 
1% l 
om potential , magnetic resistance ; 
though of course the form and the letters Rowland uses are 
different. 
The whole process of calculation of an experiment is 
embodied in the following formule. 
Let « be the deflection of the ballistic galvanometer due to 
reversal, with m induction-coils ; then 
2mSB = ka 
(S = sectional area of bar). 
Let 8 = deflection due to half turn of earth induction-coil 
(N = number of coils, A = mean area) ; then 
2NAH=«8. 
Combining these, we have . 
a NAH 
B= 3 nS 
Let G be that part of the tangent-galvanometer coefticient 
which is independent of H, so that we have for the current 
C=GH tan 6. 
Then potential 
=4arnC=4rnGH tan 6, 
magnetic resistance 
potential 
— = ee. 
G 
which is independent of H ; and 
r l 
—— 
p 
where / is the mean circumference of the ring. 
The following Tables exhibit the results of the various 
* See Phil. Mag. xvii. p. 531. 
G 2 
